(1) Field of the Invention
The invention generally relates to an encapsulation process used in semiconductor manufacturing and, more particularly, to a method of encapsulation that improves isolation of radio frequency (RF) signals in the fabrication of integrated circuits.
(2) Description of Prior Art
As integrated circuit (IC) speeds increase, seal rings have been incorporated into the device encapsulation in order to reduce radio frequency (RF) interference and signal cross coupling. The seal ring is grounded or connected to a signal ground such as a DC voltage supply line to reduce interference. In some cases, the seal ring may be part of the device packaging scheme, where a conductive lid is typically connected to the seal ring. Specific to this invention, the seal ring may be incorporated into the IC fabrication and may include a conductive covering over the substrate.
FIG. 1 shows a top view of an IC die 20 where a seal ring 22 is incorporated. The seal ring 22 is typically composed of one or more interconnected layers of conductive metal. A plurality of bonding pads 24 are shown which may be either signal inputs or outputs, or DC voltage supply and ground. A portion of the circuit containing RF circuits 26 is shown. One problem with this method is that signals from the bonding pads 24 may be capacitively coupled to the seal ring 22. This may result in unwanted signal interference appearing at one of the signal input or output bonding pads 24. In addition, interference may be coupled between the seal ring 22 and the RF circuit 26 resulting in signal distortion.
One solution is to create breaks in the seal ring to prevent propagation of the signals in the seal ring. Unfortunately, this results in a problem when water is used for cooling in dicing the die from the wafer. Hydrogen and hydroxide ions attack the low K material in the seal ring breaks, resulting in contamination to the IC structure. For instance, fluorine in fluorinated silicate glass (FSG) will react with water to form hydrogen fluoride, which will erode metal. A second solution is to increase the distance from the seal ring to the bond pads and IC circuitry. This results in an increase in die area, thereby reducing device throughput.
Other approaches employing seal rings exist. U.S. Pat. No. 5,717,245 to Pedder teaches a system using a dielectric multi-layer substrate where RF interference is reduced by grounding certain areas and encapsulating the substrate within a conductive seal ring. U.S. Pat. No. 6,028,497 to Allen et al. teaches a system where RF signals are passed through a network of holes in the base plate of the module. The holes each consist of a conductive pin surrounded by, but electrically isolated from, a conductive cylindrical shroud, thereby forming a coaxial connection. A compartmentalized seal ring attached to the top of the module segregates different circuit areas of the module. U.S. Pat. Nos. 5,864,092 and 6,105,226 to Gore et al. teach methods employing a leadless chip carrier package where a grounded conductor protrudes between input and output signal pads thereby preventing interference. U.S. Pat. No. 5,998,245 to Yu teaches a method where ESD protection is incorporated into a seal ring structure on an IC die. U.S. Pat. No. 6,028,347 to Sauber et al. teaches a method where a portion of the seal ring is formed in trenches in the semiconductor surface. An encapsulating plastic covering over the surface fills the trenches thereby preventing movement of the cover and reducing stresses due to thermal expansion. U.S. Pat. No. 5,185,654 to Mosher et al. teaches a method using a circuit carrier to encapsulate a circuit while absorbing RF and hi-voltage electro static discharge (ESD) signals. U.S. Pat. No. 4,868,716 to Taylor et al. teaches a method using a pin grid assembly with a back plane to encapsulate circuit modules and prevent RF propagation between modules. U.S. Pat. No. 6,180,876 B1 to Holmes teaches a method using grounded conductive partitions on a circuit board to isolate different circuitry.